The present invention relates to a charge control circuit for electronic instruments provided with a power device such as a solar cell and secondary batteries.
Conventional micro portable electronic instruments such as electronic wristwatches and computers employ micro silver-zinc batteries as energy sources, and it is necessary to exchange the batteries whenever the one or two year-battery life is over since the battery capacities of such batteries are small.
Accordingly, in order to reduce the need for exchanging batteries, methods for elongating the battery lives have been studied; one method being to elongate the battery life by combining the power devices such as solar cells with the secondary batteries and supplying energy from the former to the latter.
The silver-battery can be used as the secondary battery because it has the following features:
1. small size
2. excellent charge efficiency
3. small self-discharge characteristic
Although the silver-zinc battery has the features suitable for the secondary battery as illustrated above, it is necessary to select the charge conditions in order to utilize these features effectively.
In charging, for instance, if the secondary battery reaches an excess charge condition, gas is generated inside the battery and exceedingly dangerous conditions such as an expansion of the battery or, in an extreme case an explosion of the battery may take place.
The excess charge condition is easily detected by observing the terminal voltage of the secondary battery. When the silver-zinc battery becomes in the excess charge condition, the terminal voltage exceeds the normal 1.57 volts. It has been confirmed by experiment that a gas generates inside the battery when the terminal voltage is raised to approximately 1.8 volts. Accordingly, the efficient, reliable and safe charging system is completed by a charge control circuit which stops the charging when the terminal voltage of the silver-zinc secondary battery reaches 1.8 volts.